Bauxite ore digestion in the bayer process

Abstract

The process according to the invention to raise simultaneously to an optimum level the temperature and concentration of an aqueous medium intended for use in bauxite ore digestion in an alumina production plant according to the Bayer process, which includes successive phases (a) to (f) in which: a) the aqueous medium coming from digestion containing pregnant liquor is cooled and evaporated in a multiple staged flash evaporation zone; b) the sterile residue is separated, the aluminium trihydroxide is precipitated and the aqueous medium, consisting of spent liquor, intended for use in digestion is recovered; c) if need be, the required quantity of water, additional to that already evaporated in phase (a) is eliminated by evaporation in a specific evaporation section; d) the aqueous medium intended for use in digestion is heated in several stages; e) the flash vapour generated in each of the stages of the evaporation phase (a) is used in each of the stages of the heating phase (d); f) the aqueous medium intended for use in digestion coming from the heating phase (d) is heated by live steam. is characterised in that, in addition to all these phases, a simultaneous heating and evaporation phase is carried out on the aqueous medium intended for use in digestion.

What is claimed is:  
     
       1. A process line for use in a Bayer process installation for the treatment of bauxite wherein a slurry of bauxite ore in an alkaline aqueous medium is digested at temperature and pressure values to dissolve alumina as sodium aluminate and separated into solids and a pregnant liquor stream from which aluminum hydroxide is precipitated leaving aqueous medium, the process line to raise simultaneously the temperature and concentration of the aqueous medium for use in the bauxite ore digestion, including: 
       a section of interconnected evaporator-flash tanks FT 1  to FT n  installed in series and through which circulates an aqueous medium coming from the bauxite ore digestion in a digester D;  
       a section of interconnected heaters H 0 , H 1  to H n  installed in series through which circulates the aqueous medium for use in the bauxite ore digestion, each evaporator FT 1  to FT n  flash tank being connected with each heater H 1  to H n  directly associated with each flash tank FT 1  to FT n  between the flash tanks and the heaters for obtaining vapor circulation;  
       an optional specific evaporation section P for eliminating by evaporation a quantity of water, additional to that removed in the evaporator-flash tank FT 1  to FT n ;  
       at least one heater H 0  connected to a source of live steam installed at the inlet of a mixing device and/or bauxite ore digester by means of the aqueous medium to raise the temperature of the aqueous medium coming from the outlet of the first heater H 1  in the heater section H 0 , H 1  to H n  to the temperature required for digestion; and  
       a simultaneous heating and evaporation section for the aqueous medium used in the digestion.  
     
     
       2. A process line according to  claim 1  wherein said simultaneous heating and evaporation section is inserted at any point in the heating section of the heaters H 0 , H 1  to H n . 
     
     
       3. A process line according to  claim 1  wherein said simultaneous heating and evaporation section is inserted at any point in the heating section of the heaters H 0 , H 1  to H n  of the specific evaporation section P. 
     
     
       4. A process line according to  claim 1  wherein said simultaneous heating and evaporation section is inserted simultaneously at any point in the heating section of the heaters H 0 , H 1  to H n  and of the specific evaporation section P. 
     
     
       5. A process line according to  claim 1  wherein the simultaneous heating and evaporation section is inserted between heater H 0  fed with live steam, in the heating section of the heaters H 0 , H 1  to H n , and from which said simultaneous heating and evaporation section receives the aqueous medium to be treated prior to its use in bauxite ore digestion, and the digestion zone which it feeds with aqueous medium for bauxite ore digestion at the temperature and concentration required for digestion. 
     
     
       6. A process line according to  claim 1  wherein the simultaneous heating and evaporation section is inserted between heater H 0  fed with live steam in the specific evaporation section P, and from which said simultaneous heating and evaporation section receives the aqueous medium to be treated prior to its use in bauxite ore digestion, and heater H n  of the section of interconnected heaters H 0 , H 1  to H n  fed with flash vapor from the evaporator flash tanks, which it feeds with treated aqueous medium. 
     
     
       7. A process line according to  claim 1  wherein the simultaneous heating and evaporation section is inserted between heater H 1  fed with flash vapor in the heating section of the heaters H 0 , H 1  to H n  and/or in the heating section of the heaters H 0 , H 1  to the specific evaporation section P and from which said simultaneous heating and evaporation section receives the aqueous medium to be treated prior to its use in bauxite ore digestion, and heater H 0  fed with live steam in the heating section of the H 0 , H 1  to H n  and/or in the heating section of the heaters H 0 , H 1  to H n  of the specific evaporation section P, which it feeds with treated aqueous medium. 
     
     
       8. A process line according to  claim 1  wherein the simultaneous heating and evaporation section is inserted: 
       according to a first level, between heater H 1  fed with flash vapor FT 1  and from where it receives the aqueous medium to be treated, for use in bauxite ore digestion, and heater H 0  in the heating section of the heaters H 0 , H 1  to H n  and/or in the heating section of the heaters H 0 , H 1  of the specific evaporation section P fed with live steam, in the heating section of the heaters H 0 , H 1  to H n  and/or in the heating section of the heaters H 0 , H 1  of the specific evaporation section P;  
       according to a second level, between heater H 0  fed with live steam and which receives the aqueous medium at a higher temperature for use in bauxite ore digestion and;  
       either the bauxite ore digestion zone which it feeds with aqueous medium heated and/or concentrated to the level for bauxite ore digestion;  
       or, in the case of the specific evaporation section P, the heater H n  to which it feeds the treated aqueous medium.  
     
     
       9. A process line according to  claim 1  wherein the simultaneous heating and evaporation section is fed with live steam and produces vapor which is used in situ. 
     
     
       10. A process line according to  claim 1  wherein the simultaneous heating and evaporation section is connected for the use of the vapor produced: 
       to heater H 0 , combining its live steam feed with all or part of the vapor produced;  
       and/or to at least one of the heaters H 1  to H n  in the heating section and/or the specific evaporation section P;  
       and/or to one of the effects of the multiple effect of the specific evaporation section P;  
       and/or to the means for the production of hot water and/or to heat the spent liquor for use in preparing bauxite ore slurry for bauxite ore digestion.  
     
     
       11. A process line according to  claim 1  wherein the simultaneous heating and evaporation section includes at least one simultaneous heating and evaporation stage for the aqueous medium, each stage being composed of one said simultaneous heating and evaporation section or several simultaneous heating and evaporation means installed in series or in parallel. 
     
     
       12. A process line according to  claim 11  wherein the simultaneous heating and evaporation section includes at least one single stage fed with live steam and connected for treating the vapor produced, to heater H 0  combining the live steam feed of said one single stage with all or part of the vapor and/or to the exchanger of said one single stage. 
     
     
       13. A process line according to  claim 11  wherein the simultaneous heating and evaporation section includes two simultaneous heating and evaporation stages in which the first stage is connected, to treat the vapor produced, to the second stage and to heater H 0 . 
     
     
       14. A process line according to  claim 13  wherein one of stages in the simultaneous heating and evaporation section is connected upstream to the outlet of heater H 1  in the series of heaters H and downstream to the inlet of heater H 0  and in that the other stage of the simultaneous heating and evaporation section is connected upstream to the outlet of heater H 0  and downstream to the digester D when the simultaneous heating and evaporation section is installed in the section of heaters or from heater H n  when the simultaneous heating and evaporation section is installed in the specific evaporation section P. 
     
     
       15. A process line according to  claim 11  wherein the simultaneous heating and evaporation section includes three simultaneous heating and evaporation stages, in which one of the stages is connected to the outlet of heater H 0 , fed with live steam in the heating and/or evaporating sections, the other stages in series being connected upstream to the outlet heater H 1  fed with flash vapor, and downstream to the inlet of heater H 0  of the heating and/or evaporating sections. 
     
     
       16. A process line according to  claim 11  wherein one stage of the simultaneous heating and evaporation section includes a falling film simultaneous heater-evaporator which in turn includes a vertical tube exchanger, a distribution system to distribute the spent liquor on the vertical tube exchanger and a liquid-vapor separator. 
     
     
       17. A process line according to  claim 11  wherein one stage of simultaneous heating and evaporation section also includes a direct contact heater to heat the aqueous medium intended for use in digestion with vapor. 
     
     
       18. In a Bayer process treatment of bauxite where a slurry of bauxite in an alkaline aqueous medium is digested at high temperature and pressure values required to dissolve alumina as sodium aluminate and separated into solids and a pregnant liquor stream from which aluminum hydroxide is precipitated leaving aqueous medium, the improvement for raising simultaneously the temperature and concentration of said alkaline aqueous medium from which aluminum hydroxide has been separated for its use in the further digestion of bauxite, comprising the steps of 
       a) cooling and evaporating the aqueous medium in a section of interconnected flash evaporators FT 1  to FT n  installed in series;  
       b) separating a the sterile impurities residue from the pregnant liquor, precipitating the aluminum hydroxide from the sodium aluminate present in the aqueous medium, separating out the aluminum hydroxide and recovering the remaining aqueous medium for use in the further digestion of bauxite;  
       c) evaporating a quantity of water in addition to that removed in the flash evaporators FT 1  to FT n  in an evaporation section P inserted between the section for the precipitation and separation of aluminum hydroxide from the pregnant liquor and recovery of spent liquor in order to further concentrate the aqueous medium for further digestion of bauxite;  
       d) heating the aqueous medium in a section of interconnected heaters H 0 , H 1  to H n  installed in series with each flash evaporator FT 1  to FT n  directly associated with each heater H 0 , H 1  to H n ;  
       e) using the flash vapor generated in each flash evaporator FT 1  to FT n  to concentrate the aqueous medium recovered in step b) and forming the aqueous medium to be used in the digestion of bauxite, in the heaters H 0 , H 1  to H n ;  
       f) utilizing live steam supplied to a heater H 0  to raise the temperature of the aqueous medium coming from the heaters to the temperature required for digestion; and  
       g) carrying out a simultaneous heating and evaporation of the aqueous medium for use in the digestion of bauxite in a simultaneous heating and evaporation section.  
     
     
       19. Process according to  claim 18  wherein the simultaneous heating and evaporation step g) of the aqueous medium is inserted at any one point in step d). 
     
     
       20. Process according to  claim 18  wherein said simultaneous heating and evaporation step g) is inserted at any point in evaporation section P of step c) when step c) is provided with the same number of flash evaporators FT 1  to FT n  as provided in evaporation section a) and the same number of heaters H 0 , H 1  to H n  as in heating step d). 
     
     
       21. Process according to  claim 18  wherein said simultaneous heating and evaporation step g) is inserted simultaneously at any one point in steps c) and d) when step c) is provided with the same number of flash evaporators FT 1  to FT n  as provided in evaporation step a) and the same number of heaters H 0 , H 1  to H n  as in heating step d). 
     
     
       22. Process according to  claim 18  wherein the aqueous medium is fed to the simultaneous heating and evaporation step g) to be heated and/or evaporated from the heating step d) after it has been fed with live steam in heater H 0  of step d) and provides the aqueous medium for bauxite digestion at the temperature and concentration required for said digestion. 
     
     
       23. Process according to  claim 18  wherein the simultaneous heating and evaporation step g) of the aqueous medium is conducted with the aqueous medium from heater H 0  after it has been fed with live steam and the heated and/or concentrated aqueous medium is then fed to a successively arranged heater in heating step d). 
     
     
       24. Process according to  claim 18  wherein the simultaneous heating and evaporation step g) of the aqueous medium from the first stage of the heating is fed with flash vapor from step a) and said aqueous medium is then delivered to the heating step after having been fed with live steam, where it is heated and/or concentrated. 
     
     
       25. Process according to  claim 18  wherein the simultaneous heating and evaporation step g) of the aqueous medium to be heated and/or evaporated from the first stage of the heating is fed with flash vapor from step a), said aqueous medium is then delivered to the heating stage and fed with live steam whereby the aqueous medium is raised to a higher temperature, said aqueous medium is recovered from the heating stage, fed with live steam and is further heated and/or concentrated and the heated and/or concentrated aqueous medium is then fed to the digestion for digestion of further bauxite ore. 
     
     
       26. Process according to  claim 18  wherein in instances where the evaporation section P of step c) includes a multiple effect, the simultaneous heating and evaporation section of step g) is inserted in step d). 
     
     
       27. Process according to  claim 18  wherein the simultaneous heating and evaporation step g) of the aqueous medium is fed with live steam and the vapor that the simultaneous heating and evaporation produces is utilized in step g). 
     
     
       28. Process according to  claim 18  wherein the vapor produced in the simultaneous heating and evaporation step g) is fed to at least one of the heating stages H 0 , H 1  to H n  of step d) when step a) is equipped with the same number of flash evaporators FT 1  to FT n  in step a) as heaters in heating step d). 
     
     
       29. Process according to  claim 26  wherein the vapor produced in the simultaneous heating and evaporation step g) is in addition consumed in the specific evaporation section P of step c). 
     
     
       30. Process according to  claim 18  wherein the vapor produced in the simultaneous heating and evaporation step g) is in addition consumed in at least one additional heater inserted for this purpose between at least two of the heaters H 0 , H 1  to H n  in step d). 
     
     
       31. Process according to  claim 18  wherein the vapor produced in the simultaneous heating and evaporation step g) is in addition consumed to heat the spent liquor of step b) used in the preparation of bauxite ore slurry prior to the bauxite ore digestion and/or to produce water having a temperature of between 80 to 90° C. 
     
     
       32. Process according to  claim 18  wherein the simultaneous heating and evaporation step g) of the aqueous medium is carried out in at least one stage, each stage comprising one or several simultaneous heating and evaporation means operating in series or in parallel. 
     
     
       33. Process according to  claim 32  wherein the simultaneous heating and evaporation step g) of the aqueous medium is carried out in at least two stages and wherein the vapor generated by one of the simultaneous heating and evaporating stages is used to feed the simultaneous heating and evaporation stage directly preceding it. 
     
     
       34. Process according to  claim 32  wherein live steam is fed to the first simultaneous heating and evaporation stage of said simultaneous heating and evaporation step g) for heating the aqueous medium. 
     
     
       35. Process according to  claim 32  wherein one of the simultaneous heating and evaporation sections for heating and evaporating the aqueous medium of step g) is located following the final heater of step d) and another of said simultaneous heating and evaporation sections for heating and evaporating aqueous medium is inserted between the outlet of heater H 1  of step d) and the introduction of live steam into the heater H 0  of step d). 
     
     
       36. Process according to  claim 32  wherein one of the simultaneous heating and evaporation sections for heating and evaporating the aqueous medium of step g) is located at the outlet of heater H 0  and two other simultaneous heating and evaporation sections are inserted between the outlet of heater H 1  of step d) and the inlet of live steam into the heater H 0  in step f). 
     
     
       37. Process according to  claim 18  wherein in the simultaneous heating and evaporating step g), a vertical falling film of aqueous medium is used, heated by vapor, and wherein the said film generates vapor in situ which is thereafter separated from the aqueous medium resulting from the simultaneous heating and evaporation. 
     
     
       38. Process according to  claim 18  wherein aqueous medium is heated by direct contact of said aqueous medium with heated vapor injection. 
     
     
       39. Process according to  claim 38  wherein the aqueous medium intended for use in bauxite ore digestion is heated in at least one stage by heated vapor injection, and wherein the stage is located at the inlet of the simultaneous heating and evaporation step g). 
     
     
       40. Process according to  claim 38  wherein in each of the direct contact heating stages, vapor generated by each of the simultaneous heating and evaporation stages for the aqueous medium is utilized. 
     
     
       41. Process according to  claim 18  wherein the vapor produced in at least one of the simultaneous heating and evaporation stages of the simultaneous heating and evaporation step g) is compressed to raise its pressure to that of the operating pressure in the said stage of the simultaneous heating and evaporation of step g).